Age-dependent pre- and postsynaptic distribution of AMPA receptors at synapses in CA3 stratum radiatum of hippocampal slice cultures compared with intact brain

Organotypic slice cultures of rat hippocampus are widely used as experiment al preparations for the study of synaptic plasticity, but their degree of c orrespondence with intact brain is not fully known. Here, using postembeddi ng immunogold labelling, we describe the ultrastructural distribution of AM PA-type glutamate receptors (GluR1-4) in CA3 stratum radiatum of organotypi c hippocampal slice cultures at 10 days to 11 weeks in vitro and compare th e labelling with intact brain of corresponding age. In both types of prepar ation, the 11-week-old samples contained the highest proportion of AMPA rec eptor-like immunoreactive synapses. The incidence of labelled synapses, how ever, was higher in vivo (49%) than in vitro (24%). The intensity of labell ing (number of gold particles per labelled synapse) also increased with age and was also higher in vivo than in vitro. In both organotypic cultures an d intact brain, labelling was frequently found at presynaptic sites, often attached to vesicular structures. The specificity of these findings was sup ported both by light microscopic immunolabelling of GluR2/3 subunits and by electron microscopic double labelling of different epitopes of the GluR2 s ubunit. The vesicular localization of AMPA receptors was supported by Weste rn blot analysis of subcellular fractions. Morphological evidence of presyn aptic excitatory innervation of glutamatergic neurons supports a functional role for presynaptically located AMPA receptors. Our results therefore sug gest that AMPA receptors occur in both pre- and postsynaptic profiles and t hat the distribution of AMPA receptors in cultured brain slices is fundamen tally similar to intact brain, but that synaptic maturation may be retarded in vitro.